Grammar
-끼리
among ourselves; with each other
Form
Group noun or pronoun + -끼리 — “among ourselves/themselves”.
Function
After a group noun, limits the action to members of that group.
Why this form?
-와/과 함께 — “together with” adds companionship without the in-group restriction of -끼리.
How to build it
Group noun or pronoun + 끼리: 우리끼리, 자기들끼리. The particle attaches directly to its host.
Forms not to confuse
-와/과 함께
친구와 함께 갔어요. — I went with a friend.
Examples
다들 자기들끼리 소곤거리기 시작한다. — They all begin whispering among themselves.
우리끼리 비밀로 해요. — Let's keep this among ourselves.